The prevention of lowered contrast and lowered visibility caused by external light reflection or image reflection is required of image display devices, for example, cathode-ray tube display devices (CRTs), plasma displays (PDPs), electroluminescent displays (ELDs), or liquid crystal displays (LCDs). Accordingly, it is common practice to provide an antireflection laminate on the outermost surface of an image display device from the viewpoint of reducing image reflection or reflectance using the principle of light scattering or the principle of optical interference.
In image display devices, for example, liquid crystal displays, the use of an anti-dazzling laminate as one of antireflection laminates has hitherto been known for regulating optical properties to realize excellent image displays. The anti-dazzling laminate is utilized for preventing a lowering in visibility as a result of external light reflection or image reflection within image display devices. The anti-dazzling laminate is generally realized by forming an anti-dazzling layer having a concavoconvex shape on a base material. The anti-dazzling laminate is utilized for preventing a lowering in visibility as a result of external light reflection or image reflection within image display devices.
In recent years, a demand for a higher level of definition of panel resolution has led to a higher level of fineness of the concavoconvex shape of the anti-dazzling layer. Accordingly, a concavoconvex shape having a broad and large curve has been regarded as unsuitable for meeting a demand for higher definition in the anti-dazzling laminate having the above construction and thus have not been adopted. On the other hand, when increasing the fineness of the concavoconvex shape involved in higher definition of panel resolution can meet a demand for higher definition of the panel resolution. Regarding this technique, however, it has often been pointed out that external light is reflected from the display surface resulting in such a phenomenon that, for example, the image display surface is seen white (whitening), or lowered contrast.
When the anti-dazzling laminate is used on the image display surface of notebook computers and the like, a certain level of satisfactory optical properties can be provided. When the light transmitted through the backside of backlight within a display is transmitted through the concavoconvex shape face of the anti-dazzling laminate formed on the outermost surface of the panel, however, the concavoconvex shape functions as Fine lenses which disturb the displayed pixels and the like, that is, “glare,” is likely to occur. This unfavorable phenomenon makes it difficult to attain the effect of the anti-dazzling laminate per se. In particular, the occurrence of the “glare” increases with increasing the definition of the panel resolution, and, thus, effectively preventing this unfavorable phenomenon of ‘glare’ has been desired.
In order to eliminate this “glare,” for example, a method has been adopted in which surface concavoconvexes are densely provided to enhance the sharpness and, at the same time, scattering particles different from the resin for anti-dazzling layer formation in refractive index are added to, for example, impart internal scattering effect to the anti-dazzling laminate. All of proposed methods could satisfactorily solve the problem of the “glare,” but on the other hand, they sometimes lowered the visibility of the whole image. On the other hand, in the anti-dazzling laminate, the method for preventing the “glare” in high-definition panels has been regarded as a main cause of an unfavorable phenomenon, for example, a deterioration in contrast such as clouding caused by surface whitening, internal scattering effect or the like. That is, it has been regarded that ‘glare prevention’ and “contrast improvement” are in the relationship of tradeoff, and simultaneously meeting both the requirements is difficult. In the above methods, for example, black color reproduction including glossy black feeling (wet glossy black color) in on-screen display, contrast and the like have sometimes been poor. That is, gradation rendering of black color in a light room, particularly a black color gradation difference in low gradation, cannot be regarded without difficulties resulting in lowered sensitivity. Specifically, black and gray colors are only recognized as a blurred and identical color-tone black color. In particular, an anti-dazzling laminate having better anti-glare properties has a significantly lowered level of visibility.
On the other hand, in a method for preventing reflection by light interference, means for regulating the refractive index and layer thickness of each layer known in the art is, for example, to increase the refractive index of a hard coat layer having a clear and smooth outermost surface and then to provide a low-refractive index layer on the hard coat layer. According to this method, a good contrast and a lowest possible reflectance can be realized (for example, to approximately 0.1 to 0.8% in terms of reflection Y value), and reflection of an image of an external material from the surface of a display screen can be effectively prevented. This method, however, has many problems regarding production, for example, difficulties of film thickness regulation of the coating film, and many of necessary materials are expensive. Accordingly, this method is unsuitable for mass production at low cost. Although the reflectance can be minimized, in some environment where a display is viewed, image reflection cannot be satisfactorily prevented. For example, in a room having a white wall, in some cases, a phenomenon of white image reflection occurs when the surface of the white wall is smooth. Further, the reflectance can be lowered by light interference. In this case, however, interference color occurs, and, in some cases, white and black of the display screen are changed to a reddish or bluish color. On the other hand, when the reflectance is not very low, since the outermost surface is smooth, effectively preventing the image reflection is very difficult.
On the other hand, a light diffusion layer having light diffusion properties improved by bringing the ratio between the internal haze and the whole haze to 2 to 1000 and bringing the internal haze to not less than 5% has hitherto been proposed (patent document 1; Japanese Patent Laid-Open No. 295729/1999). Further, an anti-dazzling laminate is proposed in which glare preventive effect and white blurring preventive effect can be effectively attained by bringing the whole haze value to not less than 35% and not more than 50%, bringing the internal haze value to not less than 20% and not more than 40%, and bringing the internal haze value/whole haze value to not less than 0.5 and not more than 0.8 (patent document 2: U.S. Pat. No. 3,703,133). According to studies by the present inventors, however, any anti-dazzling laminate, which can prevent glare feeling and can satisfactorily reproduce glossy black feeling of images in a light room, has not been developed yet. Further, in an antireflection film formed by coating a composition comprising a binder and an inorganic filler, the brightness and resolution of transmitted images are deteriorated. To eliminate this drawback, an anti-dazzling laminate produced by forming a filler-free organic surface curing transparent film, covering a concavoconvex mat on the organic surface curing transparent film, and curing the organic surface curing transparent film (patent document 3: Japanese Patent Laid-Open No. 019301/1989) has been developed. In this anti-dazzling laminate, all of concaves and convexes are undulate although the shape is a continuous sine curve. Accordingly, no flat part is present, and, in this concavoconvex shape, desired black reproduction and glassy black feeling cannot be realized.
Accordingly, at the present time, the development of an optical laminate, which can effectively prevent the glare of an image surface and, at the same time, can realize good anti-dazzling properties and black color reproduction, especially glossy black feeling, has been desired. In particular, an optical laminate, which can be used in liquid crystal displays (LCDs) as well as in other applications such as cathode ray tube display devices (CRTs), plasma displays (PDPs), electroluminescent displays (ELDs), fluorescent display tubes, and field emission-type displays, has been eagerly desired.
[Patent document 1] Japanese Patent Laid-Open No, 295729/1999
[Patent document 2] U.S. Pat. No. 3,703,133
[Patent document 3] Japanese Patent Laid-Open No. 019301/1989